1. Field of the Invention:
This invention relates generally to electrical apparatus and more particularly to circuit breakers operable to interrupt a circuit upon occurrence of overcurrent conditions.
2. State of the Prior Art:
It is known in the prior art to provide circuit breakers to protect electrical apparatus from damage due to excessive current. Current in excess of the rated current of the circuit breaker will cause it to trip, that is, to interrupt the flow or current to the apparatus. Circuit breakers generally include two types of trip mechanisms to provide two modes of operation under varying conditions. A bimetal element, either directly or indirectly heated, provides the time-current tripping characteristic required for low to moderate overloads. Heat produced in the circuit breaker by the overcurrent condition causes the bimetal to physically deflect, releasing a trip mechanism and opening the circuit breaker contacts. The time required for the bimetal to deflect sufficiently to trip the mechanism is dependent upon the degree of overcurrent
An instantaneous trip function is provided by an electromagnetic assembly. Under extreme overcurrent conditions, such as short circuits, a magnetic field is generated, attracting an armature to release a mechanism and open the circuit breaker contacts.
The versatility of bimetal elements, however, is limited. Also precise factory adjustment of each circuit breaker is generally required to assure the proper time-current tripping characteristic. When used as a shunt trip element bimetals require large current transformers, increasing the size of the cases of circuit breakers having integral current transformers or requiring external current transformers.
Circuit breakers employing solid-state active electronic circuits are more versatile than circuit breakers employing bimetal elements. Such as circuit breaker is described in U.S. Pat. No. 3,328,639 issued to Carl E. Gryctko. Active semiconductor devices used in such breakers are susceptible to damage from surge voltages, however.
Solid state passive overcurrent devices have been used in the prior art and avoid some of the limitations of bimetal elements and active electronic circuits in trip mechanisms. Devices are described in U.S. Pat. No. 1,210,058 issued to Charles Le G. Fortescue and assigned to the assignee of the present invention, U.S. Pat. No. 2,283,706 issued to Allen Stimson, U.S. Pat. No. 3,156,849 issued to Henry David Epstein, and U.S. Pat. No. 3,172,011 issued to Charles D. Flannagan. The useful life of some prior art devices is limited, however, by a tendency to decompose.
It would be desirable to provide a circuit breaker having a trip mechanism with a solid state passive overcurrent sensing device which is compatible with existing circuit breakers of conventional design, low in cost, easy to manufacture, exhibits greater stability, and has longer life than prior art circuit breakers employing solid state passive overcurrent devices. Since operating personnel are familiar with the time-current tripping characteristic of bimetal elements, it would also be desirable to have a circuit breaker which exhibits similar characteristics.